It is known to manufacture blanks from cemented carbide, ceramic material, sintered metal or the like by means of presses. The powdered or granular material requires to be provided in such a manner that the compact, when under an applied compacting pressure, is given a homogeneous structure and allows itself to be sintered. A common forming operation is the so-called direct pressing process in appropriately designed die-sets or die-plates with which a top ram and a bottom ram are associated. In accordance with the respective compacting pressure, different densities will result for the compact. Lower-density compacts, however, while being sintered will shrink more than higher-density compacts do. An attempt is made to minimize variations in density by means of differently adjustable compression strokes for the top and bottom rams. On the other hand, in practice, varying densities may arise because of varying compressive forces which, in turn, are provoked, for example, by charging variations which may amount to some per cents with the compact heights being the same. A difficulty in manufacturing compacts, e.g. for cemented-carbide reversible cutting blades, is that a predetermined overall height is maintained between the cutting blade receptacle and at least one cutting edge which is of a predetermined distance from the cutting blade receptacle.
It has become known from DE 42 09 787 to measure the compressive force with a view to achieving a density as uniform as possible within a batch. A correction is made subsequently, which depends on the compressive force measured, via the charging volume for the compacts that succeed.
Further, it has become known from DE 197 17 217 to determine and store a desired force-stroke diagram (a desired curve), which is dependent on the geometry of the compact and the base material, for a compacting ram during compression. During compression, the values measured for the stroke and force of the compacting ram are compared to the desired curve in a computer. Using at least one separately operated portion of the compacting ram or a separate ram, the pressure acting on the material to be compacted is increased or decreased during the compression phase as soon as a deviation from the desired curve is determined to exist with a view to obtaining the same density for each compact at the end of the compression phase. This process requires that at least two position sensors be provided, i.e. for the at least one compacting ram and the at least one further compacting ram or portion of the compacting ram which provide their measuring values each to a control computer. In addition, the compacting ram or a portion of the compacting ram has associated therewith a force sensor. Its values are also input to the control computer. The position sensors are intended to ensure that if there are an upper ram and a lower ram those move to a predetermined position in the die-plate to produce the predetermined geometry of the compact and to maintain its dimensions. However, since the filling level is different variations in density might occur, which have to be avoided. This is why the further compacting ram is provided, which is actuated by the control computer when a deviation from the desired density value is determined during the respective compacting process. This process is based on the assumption that a more or less pronounced deformation may be accepted on a surface of the compact, which is required to achieve the desired density. This is the case, for instance, for reversible cutting blades on the receptacle surface.
It is understood that a procedure of this type cannot be applied to compacts if a predetermined outer contour has to be observed for the compact.
As was mentioned previously it is significant to keep to a predetermined density of a compact because its density will determine shrinkage during sintering afterwards. Naturally, density may only be determined indirectly via the compressive force applied to the material, which is known as such. Consider here that the compressive force or maximal compressive force is not to be seen completely identical to a predetermined density because the material of the compact springs back depending on its nature after a load relief so that its density will change. Such change may possibly have undesirable effects. If a certain geometry is maintained for the compact differing values of compressive forces will result in response to the filling volume, but also in response to the homogeneity of the powder in the compact.
Compacts having irregular outer contours, e.g. for reversible cutting blades including grooves or chip guide surfaces or the like on the upper surface, will exhibit differing density distributions also during the compressing operation. However, since the position and run of the cutting edge and the topography associated therewith are important an unequal distribution of density results in undesirable dimensional variations during sintering.
It is the object of this invention to provide a process for the manufacture of compacts, particularly for cemented-carbide cutting blades, by compressing metallic powder in powder presses wherein the manufacture of compacts results in articles of a reproducible accuracy in the sintering process.